The present invention refers to mobile communications based on Internet Protocol (IP), and more particularly it concerns a method of software download with session mobility support in mobile communication systems, and a mobile communication system employing the method.
Future mobile communication systems beyond the third generation (3G) will be characterised by coexistence of a plurality of access technologies such as cellular (of different generations, in particular 2nd and 3rd generation), cordless, wireless local area network (WLAN), broadcast systems, wired systems etc. These systems will be integrated on a common platform allowing them to complement each other in optimum way and to satisfy different service requirements. The various access systems will be connected to a common, flexible and seamless IP core network.
Taking into account the increasing usage of Internet-like mobile data applications in existing cellular networks, a possible evolution of such networks is towards the definition of a common transport layer based on the IP protocol. The motivation of this choice can be briefly summarised by the following points:                the core network and the radio access network have the same transport layer;        base stations from different Radio Access Technologies (RAT) can be connected to the Radio network controller by using the same transport protocol (IP);        the cellular network and Internet use the same transport protocol, thereby helping the integration of network and application domains.        
Following this approach, it results necessary to use IP mechanisms also to manage the mobility of the terminal and so to have a common set of mobility functions able to manage the terminal mobility independently of the radio access technology and in an integrated way with respect to the core network. This approach can be followed using Mobile IP (MIP) and Ipv6 mechanisms, but they don't seem appropriate for the management of micro- or pico-mobility. It is foreseen that a hierarchical solution (HMIP) and the application of different mechanisms should be integrated to resolve different mobility scenarios.
For sake of clarity, we recall here some definitions about different mobility types:                Terminal mobility: The ability of the terminal to change location, and still to be able to communicate. Terminal mobility can be:                    Discrete terminal mobility: The ability of the terminal to make discrete changes while there isn't any user data being exchanged and staying reachable for incoming calls. In the cellular context it means the support of roaming and paging procedures.            Continuous terminal mobility: The ability to change location while user data exchange may be going on. In cellular context it means the support of handover procedure. It is called seamless if the QoS is not degraded due delay or loss of data during the handover.                        Personal Mobility: The ability of end users to originate and receive calls on any terminal in any location (i.e. from different access technologies) covered by his subscription. It means that the location is performed on user based instead of terminal based.        Session Mobility: The ability of a user to change terminal. It can be split in the following categories:                    Discrete Session Mobility: The ability to change terminals while there isn't any user data being exchanged and staying reachable for incoming call. In cellular context it means to be able to reallocate the session to a new terminal and to a new network location.            Continuous Session Mobility: The ability to change terminals while user data is exchanged. To complete the scenario, we can also define:                        Service Mobility: The ability of a user to run the subscribed services irrespective of the location of the user and terminal used.        
The new cellular network will be able to provide new value added services, probably coming from Internet world and provided to the user by a multiplicity of different radio access technologies (i.e. 3G, WLAN, etc . . . ). The terminal will be able to support every type of access, that is it will be a multimode and, preferably, a reconfigurable terminal. In this scenario, the network will implement all the functions needed to support the terminal reconfiguration using in efficient way the radio resources. It is clear that, when passing from one radio access technology to another (i. e. when performing a so-called “Vertical Handover” or VHO), a multimode or a reconfigurable terminal behaves as a different terminal, so that the problem of supporting the mobility features relevant to the change of terminal is to be ensured by the network.
Among the new services, software download can be mentioned and the network must be able to define and execute the optimum strategy to perform it. The mobile industry is on the verge of introducing large-scale Internet style, media type independent, download of media objects. One of the areas of deployment for this technology is e-commerce. The most likely objects to kick off the business are ringing tones, screen savers, games and Java midlets.
In the described scenario, it may happen that a user with a multimode terminal initiates a download of a software and during the download process he has to perform a vertical-handover, e.g. from GPRS (General Packet Radio Service) to UMTS (Universal Mobile Telecommunication Service) or from a cellular network to a WLAN, for any reason (out of coverage, need to speed up the process, etc..).
Software download has generally no real-time requirement. In case of vertical handover thus a serious problem arises. The mobility management support during the software download is ensured by the network operator by means of the classical SGSN (Service GPRS Support Node) relocation procedure, which routes the information to the new terminal location. Yet this procedure manages session mobility during horizontal handovers, in which the terminal changes location under the same Radio Access Technology (RAT). In case of vertical handover, no mechanism to manage this kind of reallocation is still implemented. In this case, the download of the software should be aborted and, after the identification of the new position of the terminal, the download of the software has to start again from the beginning.
When the size of the software is small, this process has a light impact on the Quality of Service and on the extra charging of the network (i.e. more network resources to spend for the software retransmission). If the size of the software to download is huge, the implementation of mechanisms able to avoid retransmission becomes crucial to spare radio resources and speed up the download procedure. Moreover, it is possible that a class of software need to be downloaded and installed with high priority (e.g., new operating systems, new radio transmission chains, new protocol stacks, . . . ). For this class of software also it is important to avoid retransmission.
Thus, it is clear that in a short time the network architecture has to include all functionalities that are able to support efficiently reconfiguration and software download in a heterogeneous radio access context.
For the set-up and management of an IMS (IP Multimedia Subsystem) session, that is for packet oriented services that do not have real time constraints, 3GPP (3rd Generation Project Partnership) has proposed Session Initiation Protocol (SIP) as a reference protocol (see for instance 3GPP specification TS 23.228). SIP protocol is an application-layer (signalling) protocol for creating, modifying and terminating sessions with one or more participants for both unicast and multicast sessions and it has been standardised within the Internet Engineering Task Force (see e.g. IETF standard RFC 3261, which is the subject matter of the document “SIP: Session Initiation Protocol” by J. Rosenberg and H. Schulzrinne).
Use of such protocol for assisting application adaptation for IP applications during a vertical handover has been already considered, see e.g. the paper “End-to-End SIP Based Real Time Application Adaptation During Unplanned Vertical Handovers” by P. A. Pangalos et al., GLOBECOM'01, IEEE Global Telecommunications Conference, San Antonio (Tex., USA, 25-29 Nov. 2001), vol. 6, p. 3488-3493. That paper discloses practical tests and evaluation of such use for vertical handover of mobiles that are engaged in a real-time audio call. As known, real-time traffic has stringent time-delay constraints, but admits loss of some packets during handover. Thus, the suggestions contained in the paper are not suitable for non real-time software download, where packet loss cannot be admitted and may result in the need of retransmitting the whole software being downloaded. Moreover, the proposals of the paper are based on the use of the so-called “Re-invite” procedure. Such a procedure however aims at setting up a new session once the user has completed handover, and thus it is not suitable for solving the problems of interest.